We hypothesize that integrin signaling plays key roles during tumor initiation and progression. To study the role of the [unreadable]4 integrin during ErbB2-mediated mammary tumorigenesis, we introduced a targeted deletions of the [unreadable]4 signaling domain in MMTV-Neu mice. Loss of [unreadable]4 signaling delayed tumor onset and inhibited invasive growth. Ex vivo studies indicated that [unreadable]4 forms a complex with ErbB2 and enhances activation of STAT3 and c-Jun. STAT3 contributes to disrupt epithelial adhesion and promote invasion, whereas c-Jun is required for hyperproliferation. To examine the role of Focal Adhesion Kinase (FAK) in mammary tumorigenesis, we introduced a mammary gland-specific ablation of FAK in MMTV-PyMT mice. Notably, deletion of FAK completely suppressed mammary tumorigenesis. In addition, silencing of FAK induced Ras- and PI-3K-transformed mammary carcinoma cells to undergo growth arrest and senescence or apoptosis in vitro. These results suggest that [unreadable]4 promotes tumor progression by amplifying ErbB2 signaling, whereas FAK is required for Ras- and PI-3K-mediated mammary tumorigenesis. We propose: 1) To examine the oncogene specificity of the pro-tumorigenic effects of [unreadable]4 and FAK. Normal mammary epithelial cells will be altered in vitro to suppress FAK or [unreadable]4 signaling and then subjected to transformation assays with various oncogenes. Conversely, mammary tumor cells carrying distinct oncogenes will be genetically manipulated in vitro to suppress FAK or [unreadable]4 signaling and subjected to assays designed to examine their ability to proliferate, to resist apoptosis, to invade in vitro, and to form orthotopic tumors in NOD/SCID mice. Key observations will be confirmed using selected human breast cancer cell lines and transgenic mouse models;2) To elucidate the mechanisms through which [unreadable]4 signaling disrupts epithelial adhesion and promotes invasion. The mechanism by which [unreadable]4 amplifies ErbB2 signaling will be examined by using mutagenesis in combination with biochemical analysis and iRNA-mediated inhibition. The signaling pathways through which hyperactivation of Rac causes disassembly of adherens junctions will be studies by using biochemical analysis, silencing, and imaging methods;3) To study the molecular mechanisms through which FAK promotes mammary tumorigenesis. Silencing will be used to confirm that CAS mediates the pro-tumorigenic effect of FAK. Biochemical analysis of control and CAS-silenced tumor cells will then be used to identify CAS-dependent pathways. Inhibition of key downstream effectors will be used to identify the major signaling pathways through which CAS promotes mammary tumorigenesis;4) To examine the mechanisms through which [unreadable]4 and FAK cooperate to sustain ErbB2-initiated mammary tumorigenesis. Biochemical experiments will be conducted to identify signaling components jointly activated by [unreadable]4 and FAK in mammary tumor cells. Silencing will be used to test the pro-tumorigenic role of potential key signaling integrators. Mouse genetics will be used to determine if [unreadable]4 and FAK cooperate in vivo to promote ErbB2-initiated mammary tumorigenesis. PUBLIC HEALTH RELEVANE: Breast carcinoma cells evolve toward increasing malignancy in response to signals from their microenvironment, which include carcinoma-associated fibroblasts, macrophages, angiogenic endothelial cells, and the extracellular matrix that these cells produce. The integrin adhesion receptors are critical players in this signaling network. Yet, their role in breast tumorigenesis is incompletely understood. Our preliminary studies suggest that the [unreadable]4 integrin promote breast carcinoma progression by amplifying ErbB2 signaling, whereas Focal Adhesion Kinase (FAK) is required for Ras- and Pi-3K-mediated tumor initiation and maintenance. We propose to examine the mechanisms through which [unreadable]4 and FAK promote breast tumorigenesis. These studies will contribute to our understanding of the pathogenesis of breast cancer and to the identification of novel molecular targets for its therapy.